parametric analysis of no2 gas sensor based on carbon ... · tungsten halogen lamp (250 w), a...
Post on 29-Dec-2019
1 Views
Preview:
TRANSCRIPT
PHOTONIC SENSORS / Vol. 6, No. 2, 2016: 153‒157
Parametric Analysis of NO2 Gas Sensor Based on Carbon Nanotubes
Asama N. NAJE*, Russul R. IBRAHEEM, and Fuad T. IBRAHIM
University of Baghdad, Collage of Science, Department of Physics, Baghdad, Iraq *Corresponding author: Asama N. NAJE E-mail: Naje.as75@gmail.com
Abstract: Two types of carbon nanotubes [single walled nanotubes (SWCNTs) and multi walled carbon nanotubes (MWCNTs)] are deposited on porous silicon by the drop casting technique. Upon exposure to test gas mixing ratio 3% NO2, the sensitivity response results show that the SWCNTs’ sensitivity reaches to 79.8%, where MWCNTs’ is 59.6%. The study shows that sensitivity response of the films increases with an increase in the operating temperature up to 200◦C and 150℃ for MWCNTs and SWCNTs. The response and recovery time is about 19 s and 54 s at 200℃ for MWCNTs, respectively, and 20 s and 56 s at 150℃ for SWCNTs.
Keywords: SWCNTs; MWCNTs; NO2 gas; sensitivity; response time
Citation: Asama N. NAJE, Russul R. IBRAHEEM, and Fuad T. IBRAHIM, “Parametric Analysis of NO2 Gas Sensor Based on Carbon Nanotubes,” Photonic Sensors, 2016, 6(2): 153–157.
1. Introduction
Detecting gas molecules is basic to environmental monitoring, control of chemical processes, space mission, agricultural and medical
applications. The sensing of NO2 is important to monitor environmental pollution resulting from combustion or automotive emissions [1]. Nitrogen
dioxide (NO2) is flammable, colorless, and dangerous, even at very low concentration. Moreover, the interaction of NO2 and CO in sunlight
tends to produce O3, which is believed to be harmful to plants and the respiratory system of human beings and animals because of its strongly oxidizing
behavior. Therefore, from the public health and environmental protection viewpoint, the sensitive detection of nitrogen dioxide is of great scientific
importance [2]. A gas sensor is a device which detects the
presence of different gases in a region, particularly
those gases which might be harmful to humans or animals. The development of the gas sensor technology has received considerable attention in recent years for monitoring environmental pollution. It is well known that chemical gas sensor performance features, such as sensitivity, selectivity, time response, stability, durability, reproducibility, and reversibility, are largely influenced by the properties of the sensing materials used [3] .The basic principle behind gas detection is a change in an electrical property of the detecting material upon exposure to the gas. For example, in the case of carbon nanotubes, the resistance changes with exposure to different gases like NO2 and NH3. Other electrical properties of nanotubes like thermoelectric power and dielectric properties also change upon gas exposure [4]. Common gas sensors are metal oxide semiconductor such as tin oxide, zinc oxide, titanium oxide, and aluminum oxide. Problems encountered with these sensors are lack of flexibility
Received: 3 December 2015 / Revised: 24 February 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com DOI: 10.1007/s13320-016-0304-1 Article type: Regular
Photonic Sensors
154
and poor response time, and it is operated at the elevated temperature [5]. But nowadays, researchers’ goal is to develop the sensors, which can operate at room temperature and consume low power. Carbon nanotubes (CNTs) based sensors have been recently studied due to their excellent electrical, mechanical, and sensing properties. Because of the high surface area, CNTs can adsorb large amount of gases, which make them a probable contender for a gas sensor with very high sensitivity and low response time. Particularly, gas adsorption in CNTs is an important issue for both fundamental research and technical applications [6]. There are two types of carbon nanotubes’ morphology. Single-walled nanotubes (SWNTs) consist of a honeycomb network of carbon atoms and can be visualized as a cylinder rolled from a graphitic sheet. The other is multi-walled nanotubes (MWNTs) that are a coaxial assembly of graphitic cylinders generally separated by the plane space of graphite [7].
The present study focuses on the synthesis and
chemi-resistive characteristics of carbon nanotubes
(CNTs) thin film sensor. The material is prepared by
a simple chemical solution method and the thin
films are fabricated by drop-coating method.
2. Experiment
2.1 Preparation of the samples
In this work, 22 cm2 dimensions primary n-type
silicon wafer substrates were thoroughly cleaned to
de-contaminate their surface from any available
stains and dirt. A porous silicon layer (PS) was
prepared via photochemical wet etching. This
process was carried out by using ordinary light
source. Its main apparatus consisted of a Quartz
Tungsten Halogen lamp (250 W), a focusing lens
(3.8 cm) with focal length, and the diluted etching
HF acid of 50% concentration mixed with ethanol in
(1:1) ratio in a Teflon container. To prepare CNT
sample, 0.01 g of CNT was dispersed in
Dimethylformamide (DMF). A magnetic stirrer was
incorporated for this purpose for 15 minutes,
followed by 1 hour sonication. The obtained solution
was used for film deposition on porous silicon by
the drop casting method.
2.2 Gas sensor testing system
The detail of the gas sensor testing unit, which
was used in the current tests, was described
elsewhere [8]. A steel cylindrical test chamber of
diameter 163 mm and of height 200 mm with the
bottom base made removable and of O-ring sealed.
The effective volume of the chamber was 4173.49 cc,
which had an inlet for allowing the test gas to flow
in and an air admittance valve allowing atmospheric
air after evacuation. Another third port was provided
for the vacuum gauge connection. A multi-pin feed
through at the base of the chamber allowed for the
electrical connections to be established to the sensor
and the heater assembly. The heater assembly
consisted of a hot plate and a k-type thermocouple
inside the chamber in order to control and set the
desired operating temperature of the sensor. The
thermocouple sensed the temperature at the surface
of the film exposed to the analyte gas. The
PC-interfaced multi meter, of type UNI-T UT81B,
was used to register the variation of the sensor
conductance (reciprocal of resistance) exposed to
predetermined air – NO2 gas mixing ratio. The
chamber can be evacuated by using a rotary pump to
a rough vacuum of 2×102 bar. A gas mixing
manifold was incorporated to control the mixing
ratios of the test and carrier gases prior to being
injected into the test chamber. The mixing gas
manifold was fed by zero air and test gas through a
flow meter and needle valve arrangement. This
arrangement of mixing scheme was done to ensure
that the gas mixture entering the test chamber was
premixed thereby giving the real sensitivity.
3. Results and discussion
The response of a sensor upon the introduction
of a particular gas species is called the sensitivity (S).
Asama N. NAJE et al.: Parametric Analysis of NO2 Gas Sensor Based on Carbon Nanotubes
155
The most general definition of sensitivity applied to
solid state chemi-resistive gas sensors is a change in
the electrical resistance (or conductance) relative to
the initial state upon exposure to a reducing or
oxidizing gas component. It is calculated by using
the equations below [9]:
gas air
air
100 100o
R RRS
R R
(1)
or
gas air
air
100 100o
G GGS
G G
(2)
where R is the electrical resistance, G is the
electrical conductance, and the subscript “air”
indicates that background is the initial dry air state
and the subscript “gas” indicates the analyte gas has
been introduced.
Figure 1 shows the scanning electron microscope
(SEM) images for the MWCNT and SWCNTs
deposited on PS. In these figures, long nanotubes
with large agglomerates and closely packed CNTs
are shown.
(a) (b)
Fig. 1 SEM images for the CNT on PS: (a) MWCNTs and (b) SWCNTs.
The atomic force microscopic (AFM) of the
PSi/CNTs is shown in Fig. 2. Results of surface
morphology of the CNTs film had a good uniform
surface homegensity and good indication for having
nanoporous with a regular distribution of the CNTs.
The roughness average (Sa) for this layer of
PSi/CNTs was 1.04 nm while the root mean square
roughness (Sq) was 1.24 nm and ten point height (Sz)
was 5.86 nm.
Fig. 2 AFM images for the CNTs deposited on PS.
Figure 3 shows the response of the MWCNTs
and SWCNTs thin films upon exposure to NO2 gas
for mixing ratio 3% and at varying operating
temperatures. It can be seen that the response of the
films increases as the operating temperature
increases up to 200℃ and 150℃, for MWCNTs and
SWCNTs, respectively, and then decreases. The
gas-sensing response increases with temperature in
the 50℃ ‒ 200℃ and 50℃ ‒ 150 ℃ ranges for
MWCNTs and SWCNTs, respectively, because
thermal energy helps the reactions involved
overcome their respective activation energy barriers.
However, if the operating temperature becomes too
high (i.e., >150℃ for SWCNTs or 200◦C for
MWCNTs), the adsorbed oxygen species at the
sensing sites on the film surface will be diminished
and less available to react with NO2 molecules,
thereby limiting the film’s response.
As shown in Fig. 4, the sensitivity S% increases
with increasing the operating temperature T, where
Photonic Sensors
156
the maximum sensitivity of MWCNTs is 59.61%, at
200℃ of the testing temperature after which it
begins to drop with increasing T and the test is
terminated.
While the sensitivity of SWCNTs is 79.81% at
150℃ of testing temperature after which it begins
to drop with increasing T and the test is terminated.
Fig. 3 Sensitivity variation with the operating temperature of
the MWCNT and SWCNT gas sensor at 5 V bias voltage.
8004000 0
30
60
Sens
itivi
ty (S
%)
Time (s) (b)
10
70
50
20
40
200 600
200℃
150℃
100℃
50℃ 25℃
250℃
80
90
Fig. 4 Transient response of CNTs thin film at various testing
temperatures upon exposure to NO2 gas, 3% maxing ratio, at 5 V bias voltages for (a) MWCNTs and (b) SWCNTs.
At any operating temperature, the sensor
response of the SWCNTs thin film is higher than
that of the MWCNTs thin film. The results show that SWCNTs have a better performance than MWCNTs, and these results strongly depend on the structure of CNTs. When the structure of atoms in a carbon
nanotube minimizes the collisions between conduction electrons and atoms, a carbon nanotube is highly conductive. The strong bonds between
carbon atoms also allow carbon nanotubes to withstand higher electric currents than material. So when the diameter of tubs decreases the current
increases. Since SWCNTs have small diameter (1 nm‒2 nm), it will have the higher conductivity and better performances [10].
Fig. 5 Current-time variation of CNTs sensor time at 25℃, 50℃ , 100◦C, 150℃ and 200℃ testing temperature upon exposure to NO2 gas and 5 V bias voltage for (a) MWCNTs and (b) SWCNTs.
Asama N. NAJE et al.: Parametric Analysis of NO2 Gas Sensor Based on Carbon Nanotubes
157
The observed current appears in Fig. 5 increases (i.e resistance decreases) when exposing the CNT networks to NO2, which can be due to that the electron transfer occurs from CNTs to NO2 which has highly oxidizing natures. The variation of device resistance (or current) in the presence of oxidizing gas can be explained as follows: when oxidizing species like NO2 are adsorbed on the surface of p-type CNTs, the Fermi levels are shifted towards valance band, generating more holes and thus decreasing the electrical resistance.
The response time of the CNTs gas sensor decreases with increasing the operating temperature, and with the shortest response and recovery time being at about 19 s and 54 s at 200℃ for MWCNTs, respectively, and 20 s and 56 s at 150℃ for SWCNTs.
In order to explain the results, the chemical nature of the NO2 molecule is considered. Recent experimental results show that the electrical conductance of an individual semiconducting carbon nanotube strongly increases upon NO2 gas exposure and the NO2 is identified as an electron acceptor. In light of the present work, it is reasonable to propose that this behavior in the nanotube film is also due to adsorbed NO2 in the tube wall. According to recent theoretical calculations, a possible interpretation of the electrical response of CNT films to NO2 gas could be explained in terms of the physical absorption of this molecule. NO2 has an unpaired electron and is known as a strong oxidizer. Upon NO2 adsorption, a charge transfer is likely to occur from the CNTs to the NO2 due to the electron- acceptor character of NO2 molecules. The electrical response of the CNTs indicates that there is a charge transfer between the test gas and sensing element, and hence, the physisorption of gases in the nanotubes is the dominant sensing mechanism.
4. Conclusions
Thin films of two types of carbon nanotubes (SWCNTs and MWCNTs) were prepared by the drop casting method. The maximum variation of resistances to NO2 was found at an operating temperature of around 200 ℃ and 150 ℃ for
MWCNTs and SWCNTs. The response time was 19
s and 20 s for MWCNTs and SWCNTs, respectively.
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
References
[1] J. Kong, N. R. Franklin, C. Zhou, M. G. Chapline, S. Peng, K. Cho, et al., “Nanotube molecular wires as chemical sensors,” Science, 2000, 287(5453): 622‒625.
[2] X. B. Yan, Z. J. Han, Y. Yang, and B. K. Tay, “NO2 gas sensing with polyaniline nanofibers synthesized by a facile aqueous/organic interfacial polymerization,” Sensors and Actuators B: Chemical, 2007, 123(1): 107‒113.
[3] B. Ding, M. Wang, J. Yu, and G. Sun, “Gas sensors based on electrospun nanofibers,” Sensors, 2009, 9(3): 1609‒1624.
[4] S. Chopra, K. McGuire, N. Gothard, A. M. Rao and A. Pham, “Selective gas detection using a carbon nanotube sensor,” Applied Physics Letters, 2003, 83(11): 2280‒2283.
[5] M. Y. Faizah, “Room temperature multi gas detection using carbon nanotubes,” European Journal of Scientific Research, 2009, 35(1): 142‒149.
[6] S. Dhall, N. Jaggi, and R. Nathawat, “Functionalized multiwalled carbon nanotubes based Hydrogen gas sensor,” Sensors and Actuators: A Physical, 2013, 201(10): 321‒327.
[7] I. Sayago, E. Terrado, E. Lafuente, M. C. Horrillo, W. K. Maser, A.M. Benito, et al., “Hydrogen sensors based on carbon nanotubes thin films,” Synthetic Metals, 2005, 148(1): 15‒19.
[8] G. Al-zaidi, A. M. Suhail, and W. R. Al-azawi, “Palladium ‒ doped ZnO thin film hydrogen gas sensor,” Applied Physics Research, 2011, 3(1): 89‒99.
[9] L. A. Patil, A. R. Bari, M. D.Shinde, V. V. Deo, and D. P. Amalnerkar, “Synthesis of ZnO nanocrystalline powder from ultrasonic atomization technique, characterization, and its application in gas sensing,” IEEE Sensor Journal, 2011, 11(4): 939‒946.
[10] F. Y. Wu and H. M. Cheng, “Structure and thermal expansion of multi-walled carbon nanotubes before and after high temperature treatment,” Journal of Physics D: Applied Physics, 2005, 38(24): 4302‒4307.
top related